Liposome-mediated delivery of anti-neoplastic drugs such as doxorubicin has been shown to increase their therapeutic efficacy by altering the pharmacokinetics of the drug. Advances in liposome technology led to the development of small rigid polyethylene glycol (PEG)-coated liposomes that have a relatively long half-life in the circulation and a limited tissue distribution. These liposomes localize preferentially in tumors as a result of increased extravasation due to a permeable microvasculature. The development of novel Fab'-linked immunoliposomes that are targeted to the HER2 antigen resulted in specific binding and internalization of the liposomal carrier by breast cancer cells in vitro. Preclinical trials with an HER2 overexpressing tumor xenograft model in nude mice showed that anti-HER2 immunoliposomes preferentially accumulated and were retained in the tumor tissue. Anti-HER2 immunoliposomes were also found in an intracellular localization in tumor-xenograft cells suggesting endocytosis of the carrier. One barrier to liposome-mediated drug delivery is the efficiency at which the carrier releases its contents upon reaching the target tissue. The aim of the proposed work is to design liposomes that are reversibly stabilized and that can release their drug upon exposure to the acidic lumen of the endosome. This would result in an increased response at the target site and overall increase in the therapeutic index for the drug. Acid-sensitive o-citraconyl protecting groups for membrane destabilizing lipids and acid-sensitive ketal and maleyl crosslinking reagents for the release of fusion-inhibiting polymers will be utilized to prepare liposomes that are stable at neutral pH and in the presence of plasma but become destabilized in acidic organelles such endosomes. Increased drug release at the target site as a result of these strategies will result in an optimized liposome formulation that in combination with anti-HER2 targeting and incresased circulatory lifetimes afforded by PEG will result in a greater clinical effectiveness of liposomal doxorubicin in the treatment of breast cancer. Mass spectrometry will be utilized to characterize the lipidpolymer conjugates as well as any intermediates in the synthesis of these conjugates. The UCSF Mass Spectrometry Facility will aid us in obtaining this data.